ENZYME MASS TRANSFER COEFFICIENT IN AQUEOUS TWO PHASE SYSTEMS: SPRAY EXTRACTION COLUMNS

Fractional dispersed phase hold-up and overall dispersed phase side mass transfer coefficients were measured in 34, 50, 70 and 95 mm i.d. spray columns using an aqueous two phase system. Sodium sulphate-polyethyleneglycol (PEG) (4000)-water formed the aqueous two phase system and amyloglucosidase was used as a solute for the estimation of mass transfer coefficient.

The dispersed phase hold-up (e_D) and volumetric dispersed phase side mass transfer coefficient (K_D) both increased with increasing PEG-rich phase velocity. The e_Dand K_Dawere found to be independent of the column diameter and column height above a certain critical value. An increase in the phase concentration of sodium sulphate and PEG was found to reduce e_Dand K_DEmpirical and semi-empirical correlations for e_D and K_Dahave been developed.     

MULTIPHASE HYDRODYNAMICS AND SOLID-LIQUID MASS TRANSPORT IN AN EXTERNAL-LOOP AIRLIFT REACTOR—A COMPARATIVE STUDY

The solid-solid mass transfer performance of an external-loop airlift reactor was measured by dissolution of benzoic acid coated on nylon-6 particles, and the hydrodynamics of the gas-liquid-solid multiphase system in the airlift reactor were investigated. The solid-liquid system was designed to simulate the micro-carrier culture of animal cells, and some typical suspensions of immobilized enzyme particles.

The solid-liquid mass transfer coefficient remained constant below a superficial air velocity of 0.04 ms^-1 for the particles examined, but increased rapidly with further increase in gas velocity. Solids loading (0.3-3.5% w/w) did not affect the mass transfer coefficient in turbulent flow.

The mass transfer coefficient was correlated with energy dissipation rate in the airlift reactor. The mass transfer coefficient in stirred vessels, bubble columns, fluidized beds, and airlift reactors was compared.

Over an energy dissipation Reynolds number of 4-400, the solid-liquid mass transfer coefficient in the airlift device was comparable to that obtainable in fluidized beds. The performance of the airlift was distinctly superior to that of bubble columns and stirred tanks.     

MASS TRANSFER AND PHASE HOLDUP CHARACTERISTICS IN THREE-PHASE FLUIDIZED BEDS

The effects of liquid surface tension (42.6 ∼ 72,4 mN/m) and viscosity (1 ∼214mPa • sⁿ), liquid (0.01 ∼0.12m/s) and gas (0.01 ∼0.20m/s) velocities and particle sizes (1 — 8 mm) on phase holdup and mass transfer coefficient ( k_La) have been determined in a 0.142 m-I.D. × 2.0 m-high Plexiglas column. The gas phase holdup increases with liquid velocity, and the rate of increase in gas phase holdup sharply increases with gas velocity in the bed of surfactant solutions. In the beds of 1.0 and 1.7 mm glass beads, the bed contraction occurs whereas in the beds of 2.3 mm glass beads the bed contraction does not occur with an aqueous soltuion of ethanol (σ = 50.4 mN/m). The value of k_La increases with decreasing surface tension (σ ) but it decreases exponentially with increasing liquid viscosity in continuous bubble columns and three-phase fluidized beds. In three-phase fluidized beds with surfactant solutions, k_La increases with gas and liquid velocities and particle size. In three-phase fluidized beds of viscous or surfactant soltuions, k_L,a can be estimated in terms of the energy dissipation rate based on the isotropic turbulence theory and a flow regime map is proposed based on the drift flux theory.     

HYDRODYNAMICS, MIXING AND MASS TRANSFER IN A LARGE DIAMETER JET BUBBLE COLUMN

Experimental measurements for the axial and radial variations in gas holdup, axial and radial dispersion coefficients, volumetric gas-liquid mass transfer coefficient and liquid phase circulation velocity in a cone of a large diameter (122 cm) jet bubble column are presented. Two diameters of the inlet nozzle, namely 10.16 cm and 15.24 cm, three superficial gas velocities (based on cylinder diameter), 3 cm/sec, 6 cm/sec and 8 cm/sec and two superficial liquid velocities, 0.3 cm/sec and 0.6 cm/sec, are examined. The experimental data are obtained for two different bed heights.

The experimental data showed significant axial and radial variations in the gas holdup. The volumetric average gas holdup was higher at higher gas velocity and larger nozzle diameter and somewhat higher at lower liquid velocity. The axial dispersion was high while the radial dispersion was low. The volumetric gas-liquid mass transfer coefficient was larger at higher gas velocity and larger nozzle diameter. The liquid recirculation begins only at the upper end of the cone. In general, experimental data indicate that a jet bubble column provides a high degree of mixing and transport rates.     

GAS MIXING IN SLUGGING AND TURBULENT FLUIDIZED BEDS

The gas phase mixing in a fluidized bed of glass beads (d_p = 0.362 mm) in the slugging and turbulent flow regimes has been studied in a 0.1 m-ID × 3.0 m high Plexiglas column.

The gas dispersion in the downstream of the bed has been described by a diffusion process with the axial and radial dispersion coefficients. The radial dispersion coefficient of the gas phase is nearly constant with the variation of gas velocity in the slugging flow regime, but it increases with an increase in gas velocity in the turbulent flow regime.

Appreciable backmixing of the gas phase is pronounced in the slugging flow regime whereas the lower gas backmixing is produced in the turbulent flow regime. The gas backmixing coefficient increases with an increase in gas velocity in the slugging flow regime, but it decreases slightly with an increase in gas velocity in the turbulent flow regime.

The radial mixing and backmixing coefficients of the gas in terms of Peclet numbers have been correlated with the relevant dimensionless parameters (U_g/U_mf, p_s/p_g, d_p/D_t).

The gas flow pattern in the bed has been well represented by a simplified model based on the two gas phases in the dilute and dense phases which are percolating through the bed in plug flow. The present model can predict the gas exchange coefficient between the phases, the fractions of the dilute phase, the interstitial gas in the dense phase, and the interstitial gas velocity in the bed.     

HEAT TRANSFER CHARACTERISTICS OF THREE PHASE FLUIDIZED BEDS

Heat transfer characteristics of two (liquid-gas, liquid-solid) and three (liquid-gas-solid) phase fluidized beds have been studied in a 15.2 cm-ID column fitted with an axially mounted cylindrical healer. Effects of gas velocity (0-12 cm/s). liquid velocity (0-16cm/s), particle size (1.7-8.0 mm) and liquid viscosity (0.001-0.039 Pa s) on heat transfer coefficient were determined. The heat transfer coefficient increased with fluid velocities and particle size and it decreased with liquid viscosity in two and three phase fluidized beds. The bed porosity at which the maximum heat transfer occurred decreased with particle size but increased with liquid viscosity. The coefficient were correlated in terms of experimental variables. Modified Nusselt number from the present and previous studies has been correlated with modified Prandtl and Reynolds numbers.     

EVALUATION OF CONDUCTIVE HEAT TRANSFER MECHANISMS BETWEEN AN IMMERSED SURFACE AND THE ADJACENT LAYER OF PARTICLES IN BUBBLING FLUIDIZED BEDS

The evaluation of the heat transfer coefficient h_wp between a heat exchanging surface immersed in a gas fluidized bed and the adjacent layer of dense phase particles is analyzed in this contribution. Gas convective and radiant effects are not included in the present analysis.

The inclusion of h_wp, or an equivalent formation, in mechanistic models describing heat transfer has been necessary because the sudden voidage variation close to the immersed wall restrains significantly the heat transfer rate. However, there is not at present a widely accepted expression to evaluate h_wp.

A precise formulation for h_wp accounting for transient conduction inside spherical particles, the Smoluchowski effect, the concentration of particles in the adjacent layer (N_p) and an effective separation gap (l₀) is developed here.

Although N_p can be estimated, in principle, from experimental evidence in packed beds, and it is reasonably expected that l₀ = 0, the analysis of experimental heat transfer rates in moving beds, packed beds, and bubbling fluidized beds indicate that values of h_wp are, in general, smaller than expected from these assumptions. Appropriate values of l₀ and N_p are then stimated by fitting the experimental data.

The probable effect of surface asperities is also discussed by analyzing a simplified geometrical model. It is concluded that the parameter l₀ can be also effective to account for particle roughness, independently of thermal properties.     

HYDRODYNAMICS AND MASS TRANSFER IN DOWNFLOW BUBBLE COLUMN

Gas holdup, effective interfacial area and volumetric mass transfer coefficient were measured in two and three phase downflow bubble columns. The mass transfer data were obtained using the chemical method of sulfite oxidation, and the gas holdup was measured using the hydrostatic technique. Glass beads and Triton 114 were used to study the effects of solids and liquid surface tension on the gas holdup and the mass transfer parameters a and k_La. The gas holdup in three phase systems was measured for non-wettable (glass bead) and wettable (coal and shale particles) solids.

The mass transfer data obtained in the downflow bubble column were compared with the values published for upflow bubble columns. The results indicate that in the range of superficial gas velocities (0.002-0.025) m/s investigated, the values of the mass transfer coefficient were of the same order of magnitude as those observed in upflow systems, but the values of interfacial area were at least two fold greater. Also, the results showed that the operating variables and the physical properties had different influences on a and k_La in the downflow bubble column. Correlations for a and k_La for the downflow bubble column are proposed which predict the data with adequate accuracy in the range of operating conditions investigated.     

RADIAL DISPERSION AND BUBBLE CHARACTERISTICS IN THREE-PHASE FLUIDIZED BEDS

The effects of gas and liquid velocities, liquid viscosity and particle size on the radial dispersion coefficient of liquid phase (D_r) and the bubble properties in three-phase fluidized beds have been determined. A new flow regime map based on the drift flux theory in three-phase fluidized beds has been proposed.

In three-phase fluidized beds, D, increases with increasing gas velocity in the bubble coalescing and in the slug flow regimes, but it decreases in the bubble disintegrating regime. The coefficient exhibits a maximum value in the bed of small particles with increasing liquid velocity at lower gas velocities. However, it increases with increasing liquid velocity at higher gas velocities. In two and three-phase fluidized beds of larger particles (6,8 mm), D_r exhibits a maximum value with an increase in liquid viscosity at lower gas velocities, but it increases at higher gas velocities. The mean bubble chord length and its rising velocity increase with increasing gas velocity and liquid viscosity. However, the bubble chord length decreases with an increase in liquid velocity and it exhibits a maximum value with increasing particle size in the bed. The radial dispersion coefficients in the bubble coalescing and disintegrating regimes of three-phase fluidized beds in terms of the Peclet number in the present and previous studies have been well represented by the correlations based on the concept of isotropic turbulence theory.     

Heat transfer in three-phase (G/L/S) circulating fluidized beds with low surface tension media

Characteristics of heat transfer were investigated in a three-phase circulating fluidized bed whose diameter and height were 0.102 m (ID) and 2.5 m, respectively. Effects of gas and liquid velocities, particle size (0.5–3.0 mm), solid circulation rate (2.0–6.5 kg/m² s), and surface tension (47.53–72.75×10⁻³ N/m) of liquid phase on the heat transfer coefficient were examined. It was found that the heat transfer coefficient (h) between the immersed vertical heater and the riser proper of the three-phase circulating fluidized bed increased with increase in gas and liquid velocities, but did not change considerably with a further increase in liquid velocity, even in the higher range. The value of heat transfer coefficient increased gradually with increase in the size of fluidized solid particles without exhibiting the local minimum, which represented that there was no bed contraction in three-phase circulating fluidized beds due to the higher liquid velocity. The heat transfer system could attain a stabilized condition more easily with increase in particle size. The value of heat transfer coefficient increased with increase in solid circulation rate in all the cases studied due to the increase of solid holdup in the riser. The value of heat transfer coefficient decreased with increase in surface tension of liquid phase, due to the decrease of bubbling phenomena and bubble holdup. The decrease in liquid surface tension could lead to an increase in elapsed time from which the temperature difference between the heater surface and the riser became an almost constant value. The experimentally obtained values of heat transfer coefficient were well correlated in terms of dimensionless groups as well as operating variables.     

HEAT TRANSFER IN SPOUTED BEDS

Experiments were carried out in order to analyse the wall-to-bed and fluid-to-particle heat transfer coefficients in spouted Beds. wall-to-bed heat transfer coefficients were determined in cylindrical-conical and conical spouted beds for various gas flow rates, particle sizes and bed heights for spouted beds with and without draft tubes.

A new definition for wall-to-bed transfer coefficient was proposed baaed on experimental observations.

The heat tranefer area was also studied to ensure that a physically significant fluid-to-particle heat transfer coefficient was achieved.     

EFFECTS OF GEOMETRY ON HYDRODYNAMICS IN EXTERNAL-LOOP AIRLIFT REACTORS

Experimental investigations were carried out in model external-loop airlift reactors. Two reactors of laboratory scale (riser liquid height ranged between 1.16-1.56 m, riser diameter 0.03 m, A_D/A_R ratio between 0.111-1,000, total liquid volume V_T = (1.189-2.446).10^-3m³) and pilot-plant scale (riser liquid height of 4.4 and 4.7 m, respectively, riser diameter 0.200 m, A_D/A_R ratio of 0.1225 and 0.040 m, total liquid volume, V_T = (0.144-0.170) m³) were used.

The influences of reactor geometry characterized by some parameter as: A_D/A_R ratio, liquid height in riser and downcomer and liquid height in gas separator, together with the amount of introduced air, on the basic hydrodynamic design parameters: gas holdup and liquid circulation velocity were analysed.

The influence of gas sparger design on gas holdup and liquid velocity was found to be negligible.

The experimental liquid circulation velocity was correlated using a simplified form of the energy balance in airlift reactors, valid for external-loop airlift reactors with almost complete phase separation at the top.

An original dimensionless correlation for gas holdup prediction involving superficial velocities of gas and liquid, cross sectional areas, dispersion height, riser diameter, as well as Froude number, was obtained.     

SIMULATION OF TRANSPORT DYNAMICS IN FLUIDIZED-BED DRYERS

A mathematical model for predicting three-dimensional, two-phase flow, heat and mass transfer inside fluidized-bed dryers has been developed. The model consists of the full set of partial-differential equations that describe the conservation of mass, momentum and energy for both phases inside the dryer, and is coupled with correlations concerning interphase momentum-, heat-, and mass-transfer.

It is shown that the model can predict the most important engineering aspects of a fluidized-bed dryer including pressure drop, particle holdup, temperature distribution in both phases as well as drying efficiency all over the fluidized-bed. Plug-flow conditions are predicted for the gas phase, while back-mixing is predicted for the particles.

The effect of particle mass-flow-rate on fluidized-bed dryer performance is evaluated. It is shown that the lower the particle mass flow-rate, the more intense the horizontal moisture gradients, while the higher the particle rate the more uniform the moisture distribution throughout the bed.     

Drying Sawdust in a Pulsed Fluidized Bed

The objective of this work was the experimental and theoretical study of sawdust drying, in batch and continuous experiences, using a pulsed fluidized bed dryer.

In the batch experiences, a 2³ factorial design was used to determine the kinetics of drying, the critical moisture content, and the effective coefficients of both diffusivity and heat transfer, all of them as a function of the velocity and temperature of the air, the speed of turning of the slotted plate that generates the air pulses in the dryer, using sawdust with 65% moisture in each run.

In the continuous operation, a 2³ factorial design was used to study the effect of the solid flow and the velocity and temperature of the air on both the product moisture and the distribution of residence times. In order to determine these last ones, digital image processing was used, utilizing sawdust colored by a solution of methylene blue as tracer.

The statistically significant factors were the velocity and the temperature of the heating air, for both the continuous and batch operations. Although the speed of turn of the slotted plate was not significant, it was observed that the air pulses increased the movement of particles, facilitating its fluidization, especially at the beginning of drying.

The heat transfer coefficients were adjusted according to the equation Nu = 0.0014 Re_p^1.52, whose standard deviation of fit is 0.145.

The period of decreasing rate was adjusted to several diffusivity models, giving the best fit the simplified variable diffusivity model (SVDM). The curve of distribution of residence times was adjusted using the model of tanks in series, with values between 2.6 and 5 tanks.     

CARBON COMBUSTION RATES AND TEMPERATURES IN SHALLOW FLUIDIZED BEDS

The mechanism of combustion of carbon in shallow fluidized beds at temperatures 750-1000°C is studied by measuring burning rates and temperatures of spherical carbon particles ranging from 2 mm to 12 mm diameter directly in an experimental fluidized bed. Among variables investigated were inert particle size, superficial fluidizing velocity, temperature, the influence of neighbouring active particles and oxygen concentration in the fluidizing gas.

Under the experimental conditions explored, combustion was mainly kinetically controlled, so that with carbon particles larger than about 4 mm, burning rates are significantly higher than those predicted by combustion models which assume combustion to be controlled by the rate at which oxygen diffuses through a stagnant particulate phase surrounding the burning particle. The higher burning rate seems to arise because the greater mobility of particles in the bed causes the restriction to oxygen flow to the carbon surface offered by the particulate phase to be reduced and has important consequences for combustor design.

Measured carbon particle temperatures were influenced considerably by bed operating conditions ranging from 15 to 215°C higher than bed temperature.

Measured burning rates of carbon particles were found to be reduced significantly when other active particles were present in the bed. This sensitivity of burning rate to changes in active particle concentration in the bed was shown to be increasingly important once the concentration of carbon in the bed exceeded about 1%

Increasing the bed inert particle size, superficial fluidizing velocity, oxygen concentration in the fluidizing gas and bed temperature resulted in higher burning rates. The implication of these findings on combustor design are discussed.     

MAGNETICALLY STABILIZED FLUIDIZATION, PART II: CONTINUOUS GAS FILTRATION

The magnetically stabilized Auidized bed can be used as a dust filter, as was suggested earlier by Rosensweig and coworkers (1983).

In this experimental study filtration tests were carried out under various conditions. It was shown experimentally that dust particles of about one micron can be filtered effectively in beds of magnetite particles of about 300 microns, with a bed height of 10 cm. The filtration efficiency was predicted reasonably well by existing theories.

By using a magnetic grid plate, proposed by Jaraiz and coworkers (1983), an even quasi continuous downflow of the solids can be realized. Since it was shown that the dust penetrates the bed for a few millimeters only, a very effective removal of the collected dust can be obtained. In this manner, the filter was used effectively in a continuous operation.

The results appear to be especially promising for dust filtration of gas flows under high pressure, and at elevated temperatures, limited only by the Curie point of the bed material.     

AN EXPERIMENTAL STUDY OF PHASE DISTRIBUTION AND TURBULENCE STRUCTURE FOR SOLID/LIQUID FLOW IN A HORIZONTAL PIPE

Local measurements of the velocity and phase distribution were made for solid/liquid two-phase flows in a horizontal pipe using a multidimensional laser Doppler anemometer (LDA) system. The pipe used had a 30.6 mm inner diameter and was made of a special optically clear material, Fluorinated Ethylene Propylene (FEP), which has the same index of refraction as water. The test section and laser probes were submerged in water so that no optical corrections were necessary. Simultaneous measurements of the liquid and particle phases were obtained and discriminated based on differences in the residence time that each particle had when crossing the LDA's measurement volume.

Positive buoyant and negative buoyant solid spherical particles, about 2 mm in diameter, were used in this study. A complete data set of the phasic mean velocities, turbulence and volume fractions were taken for each particle type.

The results presented herein are complete and self-consistent, and can be used to assess the predictive capabilities of multidimensional two-fluid computational fluid dynamic (CFD) models.     

SUBLIMATION OF PURE SUBSTANCES IN GAS FLUIDIZED BEDS AT ATMOSHPERIC PRESSURE.

This paper deals with the sublimation of large bodies, or “objects”, made up from a pure substance in a bubbling gas fluidized bed of considerably smaller particles, or “fines”. The influence of such parameters as the gas velocity, the bed temperature, the size and the adsorption capacity of the fines has been investigated.

The results obtained clearly show that the rate of sublimation in fluidized beds is far higher than in air alone. It increases with increasing bed temperature, decreasing particle size, increasing powder mass capacity, and roughly varies as a parabolic function of time. It has also been observed that the temperature difference between the bed and the object surface, or “temperature depression”, depends on the fines characteristics as well as on bed temperature, but is independent of gas velocity when good solid mixing conditions are achieved.

Bed-to-object heat and mass transfer coefficients have been deduced from data points and attempts have been made to provide a reasonable theory to account for them. After a complete examination, the idea of interpreting transport phenomena based on a well-adapted “surface renewal model” has been proposed.     

Salt Effects on Single Aqueous Drops Falling through an Immiscible Organic Liquid

Drop settling (or rising) is crucial to the breaking of liquid-liquid dispersions. Most of the empirical correlations for terminal velocity of drops are formulated based on experimental data for systems of dispersed organic phase and continuous aqueous phase using pure and nearly pure liquids. Because the presence of the various third components in one or both of the phases mayaffect the settling of the drops, the terminal velocity and the onset of oscillation of aqueous drops containing various concentrations of electrolytes falling through n-dodecane were studied in this work. The applicability of existing empirical correlations to these reversed systems of continuous organic phase and dispersed aqueous phase was also examined.

Dissolved electrolytes have the effect of causing the terminal velocities and their peak velocities at the onset of drop oscillation to become faster and the peak diameters to become smaller when their concentrations are high enough to cause sufficient changes of physical properties. Existing correlations for drop terminal velocity in pure systems are shown to be adequate in the presence of electrolytes. For high interfacial tension systems, the Hu-Kinter correlation predicts the terminal velocity of organic drops in water and aqueous drops in organics equally well, independent of the presence of electrolytes.

The criteria of Hu-Kintner can be used to estimate the peak velocities and peak diameters at the onset of oscillation within a deviation of 3.7%.     

ATMOSPHERIC CONTACT DRYING OF GRANULAR BEDS WETTED WITH A BINARY MIXTURE

When regarding the atmospheric contact drying of granular beds wetted with a liquid mixture, both the drying rate and the selectivity of the process, i.e. the change of moisture composition, are of interest. The batch drying of a free flowing ceramic substance, wetted with a 2-propanol-water mixture, is investigated in a rotary dryer with heated wall and air flow.

The theoretical analysis is based on physical models for heat and mass transfer, moisture migration and particle transport, which are presented in examples.

The experimental and theoretical results show that higher selectivities can be achieved by reducing the particle size because of the lower liquid-phase mass-transfer resistance. An increase of the rotational speed leads to a higher drying rate with slightly decreased selectivity if the particles are sufficiently small, since contact heat transfer is enhanced.